1. Field of the Invention
The present invention relates to a method of wire bonding two conductors, a method of forming a bump and a bump, and more particularly to a method of wire bonding a first conductor and a second conductor by first bonding on the first conductor and second bonding on the second conductor, wherein a bump of a preferable shape is formed on the second conductor by ball bonding in advance, so that the wire-bonded wire does not come into contact with another wire or with a surface of a substrate, a method of forming such a bump and a bump.
2. Description of the Related Art
At the time of executing wire bonding, a method of wire bonding a semiconductor chip and a conductive lead or contact area, using a capillary with an Au wire inserted through the capillary, wherein a ball is formed at the tip end of the Au wire projected from the capillary by discharge from an electrode of an electric flame off, and first bonding with the capillary located on the semiconductor chip and second bonding with the capillary moved onto the contact area are carried out, is used commonly.
In this method, in the case where the contact area is made of a material having a poor bonding property with respect to the Au wire, such as Cu, Ni and flash Au plating, bonding cannot be executed directly on the contact area. For this reason, a base such as Au plating, an Au thick film or the like is previously formed on a part of the contact area onto which bonding is to be carried out.
However, it is not preferable to provide such a base in practice, and thus a method capable of directly bonding on the contact area even in the case where the contact area is made of a material having a poor bonding property with respect to the Au wire has been demanded.
For example, as shown in FIG. 10, Japanese Patent Application Laid-Open Publication No. 10-112471 discloses a method of wire bonding a first conductor 10 and a second conductor 20 by forming a bump 51a by ball bonding on the second conductor 20 of a substrate 30, wedge bonding the bump 51a at a position on a side thereof away from the first conductor 10, first bonding on the first conductor 10, and looping a wire 50 from the first conductor 10 to the bump 51a, followed by second bonding on the bump 51a. 
According to this method, since wedge bonding is executed on the second conductor on the rearward side of the bump or on the side of the bump away from the first conductor after formation of the bump by ball bonding, a wedge bond has a bent shape so that generation of a bump tail can be eliminated.
In addition, it is disclosed that in second bonding on the bump, as shown in FIG. 11, when a capillary 40 is located such that a bent part 51b of the wire 50 extending from the bump 51a is placed in a bore 41 of the capillary 40, the bent part 51b is pressed by one side of an inner wall of the bore 41 so that the wire 50 and the bent part 51b are bonded together in the state where the wire 50 is deformed by the other side of the inner wall of the bore 41.
However, according to the method disclosed in this publication, as shown in FIG. 11, the wedge bond after the bump formation and the wire are bonded together, that is, curved surfaces are bonded to each other. Therefore, in the case where bonding is carried out while the curved surfaces are misaligned, consequently bending of the wire is generated, leading to a possibility of undesirable contact between adjacent wires.
Moreover, although generation of a bump tail can be restrained by forming the wedge bond after the bump formation to have a shape curved rearwards, a sufficient inclination and a flat surface cannot be ensured in bonding the wire to the bump, resulting in contact of the wire 50 with the substrate 30 or the contact area, as shown in the broken line in FIG. 11, which contact is generated after bonding of the bump 51a and the wire 50, being not prevented sufficiently.
The present invention has been made in view of the foregoing disadvantages of the prior art.
Therefore, an object of the present invention is to provide a wire bonding method, a bump forming method and a bump which are capable of preventing contact of a wire with a substrate or a conductive lead after bonding the wire to a bump, and which are capable of preventing generation of bending in the wire during bonding the wire to the bump.
In order to solve the above-mentioned problems, the following means are employed in the present invention.
According to a first aspect of the present invention, there is provided a method of wire bonding a first conductor and a second conductor. The method comprises the steps of: forming a bump on the second conductor by ball bonding on the second conductor; forming an inclined surface on an upper end of the bump; first bonding one end of a wire to the first conductor; and looping the wire from the first conductor to the bump on the second conductor, and then second bonding the other end of the wire to the inclined surface of the bump.
According to this aspect of the present invention, there is also provided a method of wire bonding a first conductor and a second conductor using a capillary having a bore formed therein and a wire extending through the bore. The method comprises the steps of: forming a bump on the second conductor by ball bonding on the second conductor while positioning the capillary above the second conductor; forming an inclined surface on an upper end of the bump by moving the capillary upwards, laterally moving the capillary to a side of the bump away from the first conductor, and moving the capillary downwards, followed by cutting the wire; first bonding one end of the wire to the first conductor; and second bonding the wire extending from the bore of the capillary and the inclination surface of the bump by means of a lower end portion of the capillary while positioning the capillary at the inclination surface of the bump, and cutting the wire at a portion thereof near the bonded portion.
In a preferred embodiment of the method of wire bonding according to the present invention, the inclined surface of the upper end of the bump is formed to be flat or concave; and the second bonding step includes pressing the wire onto the inclined surface of the bump by means of the lower end portion of the capillary while positioning the capillary at the inclination surface of the bump.
In a preferred embodiment of the method of wire bonding according to the present invention, the inclined surface of the bump is so formed that a line connecting opposite edges of the inclined surface has an inclination angle of 2xc2x0 to 60xc2x0 to a face of the second conductor, which angle is open toward the first conductor.
According to a second aspect of the present invention, there is provided a method of forming a bump on a second conductor in advance by ball bonding when a first conductor and the second conductor are wire bonded by carrying out first bonding on the first conductor and carrying out second bonding on the second conductor. The method comprises the steps of: lowering a capillary onto the second conductor and ball bonding a ball formed at a distal end of a wire to the second conductor, to thereby form a bump on the second conductor; then vertically raising the capillary; thereafter laterally moving the capillary in a direction away from the first conductor; subsequently lowering the capillary onto the bump to press a lower end portion of the capillary against the bump; and thereafter laterally moving the capillary in a direction toward the first conductor, whereby the bump is provided on an upper end thereof with an inclined surface.
According to this aspect of the present invention, there is also provided a method of forming a bump on a second conductor in advance by ball bonding when a first conductor and the second conductor are wire bonded by carrying out first bonding on the first conductor and carrying out second bonding on the second conductor. The method comprises the steps of: lowering a capillary onto the second conductor and ball bonding a ball formed at a distal end of a wire to the second conductor, to thereby form a bump on the second conductor; then vertically raising the capillary; thereafter laterally moving the capillary in a direction toward the first conductor; then vertically raising the capillary; thereafter laterally moving the capillary in a direction away from the first conductor; subsequently lowering the capillary onto the bump to press a lower portion of the capillary against the bump; and thereafter laterally moving the capillary in a direction toward the first conductor, whereby the bump is provided on an upper end thereof with an inclined surface.
In a preferred embodiment of the method of forming a bump according to the present invention, the inclined surface of the bump is so formed that a line connecting opposite edges of the inclined surface has an inclination angle of 2xc2x0 to 60xc2x0 to a face of the second conductor, which angle is open toward the first conductor.
According to a third aspect of the present invention, there is provided a bump which is formed on a second conductor in advance by ball bonding when a first conductor and the second conductor are wire bonded by carrying out first bonding on the first conductor and carrying out second bonding on the second conductor. The bump comprises an upper surface, which is formed to be flat or concave and which is so formed that a line connecting opposite edges of the upper surface has an inclination angle of 2xc2x0 to 60xc2x0 to a face of the second conductor, which angle is open toward the first conductor.